Driving mechanisms of the in-wheel motor vehicle fall into two broad categories: a mechanism for a direct drive motor (Direct drive motor scheme) and a mechanism for a geared motor (Geared motor scheme). The direct drive motor scheme, in which an output shaft of a motor is directly coupled to a drive shaft (hub) of a wheel, includes no speed reducer, and thus can be designed with a small number of parts and to have a reduced weight. The geared motor scheme, in which a speed reducer is provided to reduce the speed with which an output of the motor is transmitted to a drive shaft of a wheel, can increase the torque of the motor using the speed reducer, and thus can be designed to have an increased output efficiency so as to improve the dynamics of the vehicle while keeping the motor smaller in size.
The direct drive motor scheme, in which the torque is increased by providing a longer torque arm in the motor, would disadvantageously require a motor larger in size and impair the output efficiency and the dynamics of the vehicle.
On the other hand, the geared motor scheme, in which a speed reducer is placed between the wheel and the motor, would disadvantageously require a space for accommodating the speed reducer and accessorial devices for supplying gear oil, thus impeding the miniaturization of the motor as a whole.
For example, the in-wheel motor as disclosed in JP 8-289501 A and JP 2004-114858 A has a relatively great dimension in an axial direction of a wheel, and the motor is designed to project from the wheel. If the in-wheel motor designed to have the motor or other components projecting from the wheel is used as a steerable wheel, the mechanism for use in steering operation could possibly interfere with the motor or other components that project from the wheel. In addition, a portion projecting from the wheel would occupy a space within the vehicle, and thus would reduce the greatest intended advantage of increased available space within the vehicle.
The present invention has been made against this backdrop. It would be desirable to provide a wheel rotating device for an in-wheel motor vehicle, in which a motor, a speed reducer and other driving systems are packed compactly so that an available space within the vehicle can be as large as possible.